|Scott, Russell - Russ|
Submitted to: Journal of Arid Environments
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/5/2007
Publication Date: 3/6/2007
Citation: Nagler, P.L., Glenn, E., Kim, H., Emmerich, W.E., Scott, R.L., Huxman, T., Huete, A. 2007. Relationship between evapotranspiration and precipitation pulses in a semiarid rangeland estimated by moisture flux towers and modis vegetation indices. Journal of Arid Environments. 70:443-462. Interpretive Summary: Evapotranspiration (ET), the evaporation from the soil surface and plants is a major component of the land surface water budget in arid and semi-arid watersheds. There is a great need to develop methodologies to predict ET over entire watersheds; however measurements at this scale are not available. This study applies a method that uses information readily available from satellites to estimate ET over a semiarid watershed in southern Arizona. The method is shown to accurately reproduce estimates made from ground instruments and it is used to estimate ET across the watershed. The results of this study suggest that this relatively simple approach might provide accurate and cost effective ET estimates in basins with similar climate and vegetation.
Technical Abstract: We used moisture Bowen ratio 'ux tower data and the enhanced vegetation index (EVI) from the moderate resolution imaging spectrometer (MODIS) on the Terra satellite to measure and scale evapotranspiration (ET) over sparsely vegetated grassland and shrubland sites in a semiarid watershed in southeastern Arizona from 2000 to 2004. The grassland tower site had higher mean annual ET (336 mm yr 1) than the shrubland tower site (266 mm yr 1) (Po0.001). ET measured at the individual tower sites was strongly correlated with EVI (r ¼ 0.80–0.94). ET was moderately correlated with precipitation (P), and only weakly correlated with net radiation or air temperature. The strong correlation between ET and EVI, as opposed to the moderate correlation with rainfall, suggests that transpiration (T) is the dominant process controlling ET at these sites. ET could be adequately predicted from EVI and P across seasons and tower sites (r2 ¼ 0:74) by a single multiple regression equation. The regression equation relating ET to EVI and P was used to scale ET over 25 km2 areas of grassland and shrubland around each tower site. Over the study, ratios of T to ET ranged from 0.75 to 1.0. Winter rains stimulated spring ET, and a large rain event in fall, 2000, stimulated ET above T through the following year, indicating that winter rain stored in the soil pro'le can be an important component of the plants’ water budget during the warm season in this ecosystem. We conclude that remotely sensed vegetation indices can be used to scale ground measurements of ET over larger landscape units in semiarid rangle- lands, and that the vegetation communities in this landscape effectively harvest the available precipitation over a period of years, even though precipitation patterns are variably seasonally and interannually.